Avian virus agility - what's their strategy?

With bird flu making the headlines again, despite the low risk to humans, it’s clear that viruses carried by birds are a problem. Avian influenza refers to the disease caused by influenza A viruses (IAVs) and occurs naturally in wild aquatic birds. These viruses also have the ability to adapt and infect other species, including domestic poultry and mammals. Another major concern is their potential to become a danger to humans after building the ‘agility’ to spread from one person to another.​​​​​​​

Caroline Chauche and CVR Principal Investigator Pablo Murcia have worked with collaborators from Cornell University, the University of Rochester and the Weipers Centre Equine Hospital - taking a closer look at the strategies used by IAVs to ‘jump’ from birds into mammals. Their findings are published this week in the Journal of Virology. Here we ask Pablo to explain this research in more depth:

How would you summarise this research?

‘Influenza A viruses (IAVs) cause significant burden of disease in humans and animals. Wild birds are the natural reservoir of IAVs, and it is from them that IAVs jump the species barrier to establish infections in new hosts.

Mammals, such as humans, pigs and horses, are commonly exposed to avian influenza viruses but only in rare cases do these viruses adapt to mammals and establish as new ‘non-avian’ viral strains. Examples of avian-origin mammalian viruses include ‘H3N8’ equine (horse) influenza virus, ‘H3N2’ canine (dog) influenza virus, and the Eurasian ‘H1N1’ swine (pig) influenza virus. It is still disputed whether the IAV that caused the devastating 1918 pandemic 100 years ago originated directly from birds. The H and N numbers refer to the structural proteins on the virus particle surface and are used to classify IAVs. For instance, the IAVs responsible for this year’s influenza season are H3N2 and H1N1.

To better understand how avian-origin IAVs adapt to mammals we asked whether one exemplary virus, the H3N8 equine influenza virus (EIV), changed significantly over time. To this end, we examined the molecular changes that appeared on the non-structural protein 1 (NS1) over a period of 50 years (from 1963 to 2013). We focused on this viral protein because it is widely known for its ability to fight the innate immune response of the host, which is a key barrier to some viruses being able to infect a new species. NS1 is called non-structural because it is not considered a major component of influenza virus particles. But this doesn’t mean that it isn’t important during infection.

We infected horse cells with a more recent ‘horse-adapted strain’ (that is, a virus that circulated in horses for over 40 years and likely has evolved specifically to infect horses) and with a more "avian-like" virus (one of the first EIVs isolated in 1963) and compared how the horse cells responded to these infections at a very high level of detail. By comparing the outcomes of infection (such as virus growth, numbers of infected cells, viral and host cell RNA and protein production) we were able to show that the adapted EIVs did indeed use different strategies to overcome the innate response of equine cells.

This was associated with changes in the viral NS1 protein. To test the hypothesis that these NS1 changes were important, we then produced genetically-engineered EIVs with the observed NS1 mutations that appeared during EIV evolution and repeated all our experiments again. This allowed us to identify the precise mutations responsible for the differences in horse cell response and outcomes of infection. Interestingly, through evolutionary analysis we could show that such NS1 mutations appeared over the years in a stepwise fashion and as soon as they appeared they became permanently fixed in the genome of all EIVs very rapidly, indicating that they provided ‘high fitness’ to the virus or that they made it more likely that a EIV carrying said mutations would spread better in the horse population.

What’s new about your findings?

Our findings provide insight on the role of evolution on virus emergence and the strategies that avian viruses employ to adapt to mammals. This is of particular importance as avian influenza viruses such as H5N1 and H7N9 are able to infect people often causing fatal disease. However, as yet, these particularly dangerous viruses appear not to have been able to adapt to spread in humans. This means that a person may become infected but this won’t lead to an outbreak among a wider population. Our study might provide knowledge of what a virus might need to overcome to jump into the human population and create a health concern.

What’s the long term impact of your findings and what will happen with this piece of research?

It would be good to know if our findings also applied to other avian-origin IAVs that have found a way to adapt to mammals. Also, influenza adaptation is the result of changes in various viral proteins, not just NS1. Therefore, further studies on the functional evolution of other viral proteins are needed to understand the complex rules that govern virus emergence.